Add maxConcurrent parameter to flatMapIterable

[lukaciko]

Refs #3711. Should these new overloaded functions be annotated with @Beta?

[lukaciko]

The failing test (testNoMoreRequestsAfterUnsubscribe) seems unrelated. All tests pass locally.

[akarnokd]

If you could add one unit test for each method that verifies maxConcurrent is honored, that would be great.

[lukaciko]

While writing the test I figured you might have been right when commenting on issue #2627. If I am not mistaken all the observables created with OperatorMapPair.convertSelector use the same scheduler (there is no way to subscribe them to different schedulers) and thus are run sequentially and until completion. Would adding a Scheduler parameter make sense?

[akarnokd]

There is no scheduler involved here so no.

[akarnokd]

👍 as it is now

[stevegury]

👍